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What's wrong with the hollow sway bars? I have been tracking my Cobb/Hotchkis hollow bars front and rear for many years and I never seen or hear of any problems.

Maybe that person was abusing the car to the way that the sway bar wasn't design to do??? If they were installed properly and have them maintained, that shouldn't happen at all unless it's a defective one.

Could have been defective - the break happened right inside the bracket - perhaps newer revisions have been made - I would just keep an eye on them and keep them free from rust

I guess the reason why I never had any problem is the fact that my spring rates in the front depending on the track is between 500-900 lb/in and rear is 400-700. With this stiff springs rate, the sway bar don't twist as much and therefore, less load on the bar.

In my case with running very stiff spring rate for the race track, the hollow bar will be a benefit for me for weight purpose. But for street use, I would recommend solid bar for sure.

Conclusion:
Tube torsion members can be used successfully as swaybars with some benefit in weight reduction. As an example, 2 individual, rate equivalent (approximately), common sized generic bars 1100mm long with 300mm blades, one solid 24mm the other tube of 25.4 x 3.96mm (1” x 0.156”) gives a weight reduction of about 2.5kg with about 200g of that as unsprung
mass per wheel.

However the downside to these is the increase in stress levels for equal OD or rate, and therefore reduced strength. Also the complications seen in manufacture swaybars from tubular medium, and the importance of geometric control under bending make these more difficult and expensive to make. The attachment points at the blades can also be a problem with these swaybar designs.

The additional complications and downsides of hollow bars seem to outweigh their advantage in overall weight and unsprung mass, which can be seen to only be marginal in street car application. Even OE manufacturers rarely use this type of design in their
swaybars. However with the right conditions and setups, they can bring some gain in racing situations, which can require weight reductions to the gram.

Lol. I said "the center of the rod doesn't do much except add weight" because, as the white paper shows, the outer diameter carries the loads. That's why the stiffness of a solid rod or tube of the same overall diameter is about the same.

And linked to the white paper so you could read it and decide for yourself what to buy. I've had both, and never had any problems.